Embodiments of the invention relate generally to hybrid and electric vehicles and, more particularly, to transferring energy from one energy storage device (ESD) to another ESD of a traction system.
A hybrid electric vehicle (HEV) may combine an internal combustion engine and an electric motor powered by an energy storage device, such as a traction battery, to propel the vehicle. Typically, the electric motor of an HEV is coupled between the internal combustion engine and the transmission to take advantage of the torque increase through the transmission. Such a combination may increase overall fuel efficiency by enabling the combustion engine and the electric motor to each operate in respective ranges of increased efficiency. Electric motors, for example, may be efficient at accelerating from a standing start, while combustion engines may be efficient during sustained periods of constant engine operation, such as in highway driving. Having an electric motor to boost initial acceleration allows combustion engines in HEVs to be smaller and more fuel efficient.
A purely electric vehicle (EV) typically uses stored electrical energy to power an electric motor, which propels the vehicle. EVs may use one or more sources of stored electrical energy and are configured to use energy from an external source to recharge the fraction battery or other storage devices. For example, a first source of stored energy (sometimes referred to as an “energy” source) may be used to provide longer-lasting energy while a second source of stored energy (sometimes referred to as a “power” source) may be used to provide higher-power for, for example, acceleration from standstill or boost during operation. First and second sources may include chemical-based batteries or may include ultracapacitors, as examples. Typically, the source(s) of electrical energy (energy and/or power batteries) in EVs are charged via a plug-in charger or other external energy source. With typically complete reliance on plug-in power, an EV may have increased energy storage capacity as compared to an HEV.
A plug-in hybrid vehicle (PHEVs) may include both an internal combustion engine and an electric motor powered by an energy storage device, such as a traction battery. Typically a PHEV is configured to use energy from an external source to recharge the traction battery or other storage devices. Thus, with increased reliance on plug-in power, a PHEV may have increased energy storage capacity as compared to an HEV.
There are generally two types of PHEV: parallel and series. In a parallel PHEV arrangement, the electric motor is coupled between the internal combustion engine and the transmission, enabling the combustion engine and the electric motor to each operate in respective ranges of increased efficiency, similar to an HEV. In a series PHEV arrangement, the electric motor is coupled between an energy storage device and the vehicle drive axle, while the internal combustion engine is coupled directly to the energy storage device and not to the vehicle drive axle. The series PHEV may also be referred to as an extended range electric vehicle (EREV), in reference to a purely electric drive system having energy augmentation to the energy storage system via the internal combustion engine and via, for instance, a liquid fuel storage system.
In general, EVs, HEVs, and PHEVs typically include regenerative braking to charge the charge storage devices during braking operations. Also, such vehicles may include on-road and off-road vehicles, golf cars, neighborhood electric vehicles, forklifts, and utility trucks as examples. These vehicles may use either off-board stationary battery chargers or on-board battery chargers to transfer electrical energy from a utility grid or renewable energy source to the vehicle's on-board traction battery.
Most such vehicles have two ESD packs, a high voltage (HV) ESD pack for the drive train and a low voltage (LV) ESD pack for auxiliary functions such as lighting and vehicle control systems. The HV ESD pack is often interfaced to the DC link coupled to the drive train using one or more DC-DC boost converters configured to boost the HV ESD pack voltage to a higher voltage for driving the drive train. In some systems, the LV ESD pack is charged from the HV pack using an additional step-down DC-DC converter. This additional DC-DC converter, however, adds cost and weight to the vehicle.
It would therefore be desirable to provide a system for charging the LV ESD pack from the HV ESD pack using existing components to minimize the use of additional components in the system.